1. Field of the Invention
The present invention relates to an anisotropic conductive film, a method for mounting a semiconductor chip, and a semiconductor device, and more particularly, to an anisotropic conductive film which appropriately mounts a semiconductor chip to a substrate with an active element formation surface of the chip facing the substrate, a method for mounting a semiconductor chip, and a semiconductor device.
2. Description of Related Art
Anisotropic conductive films are typically employed for so-called flipchip mounting, in which a semiconductor chip is mounted with the surface thereof bearing electrodes looking downward. An anisotropic conductive adhesive is formed in a sheet, and is called ACF (anisotropic conductive film). The thickness of the ACF is 50 xcexcm approximately. The ACF has an elongated tape-like configuration, and before use, a cover film remains attached on both sides of the ACF.
The ACF is typically produced by applying electrically conductive particles, such as epoxy resin particles plated with a metal such as Ni or Au, to an epoxy-based or polyurethane-based thermosetting resin. Also occasionally in use are metal-coated resin particles which are further coated with a resin. In such a case, when the resin particles are in contact with each other or in contact with a bump of a semiconductor chip, the surface resin coatings are destroyed, assuring electrical conduction therebetween.
A typical mounting method of a semiconductor chip employing a conventional anisotropic conductive film is now discussed. FIGS. 8(A) and 8(B) are cross-sectional views showing a semiconductor chip using a conventional anisotropic conductive film. FIG. 8(A) is the cross-sectional view showing the semiconductor chip that is thermo-compression bonded through the conventional anisotropic conductive film, and FIG. 8(B) is the cross-sectional view showing the semiconductor chip that is mounted on a warped substrate using a conventional anisotropic conductive film. Referring to FIG. 8(A), the anisotropic conductive film 3 is glued onto the substrate 2 having a wiring pattern 21 thereon. The semiconductor chip 1 is mounted on the anisotropic conductive film 3 with bumps 11 formed on electrodes of the semiconductor chip 1 facing the wiring pattern 21. Using a thermo-compression tool 51, the semiconductor chip 1 is heated from the surface thereof opposite to the surface bearing the bumps 11 while being pressed in the direction of an arrow A.
When heated, the anisotropic conductive film 3 gains fluidity, filling the space surrounding the bump 11 and the wiring pattern 21, and furthermore flows out of bonding surfaces between the semiconductor chip 1 and the substrate 2, and clings to the sides of the semiconductor chip 1. Some of electrically conductive particles 36 are clamped between the bump 11 and the wiring pattern 21.
After the thermo-compression process, when the setting of the anisotropic conductive film 3 is completed, the semiconductor chip 1 and the substrate 2 are bonded through the anisotropic conductive film 3. The anisotropic conductive film 3 clinging to the sides of the semiconductor chip 1 forms a fillet 34, reinforcing the mechanical bond between the semiconductor chip 1 and the substrate 2. The electrically conductive particles 36 clamped between the bump 11 and the wiring pattern 21 serve as an electrically conductive medium.
The above-referenced conventional art suffers from the following drawback.
When the fluidity of the anisotropic conductive film 3 is relatively large, the anisotropic conductive film 3 easily flows and clings not only to the sides of the semiconductor chip 1 but also to the thermo-compression tool 51, as represented by a deposit 35 as shown in FIG. 8(A), when the semiconductor chip 1 is heated and pressed by the thermo-compression tool 51. The management of the steps involved in the thermo-compression process of the semiconductor chip increases if part of the anisotropic conductive film 3 frequently clings to the thermo-compression tool 51, the thermo-compression tool 51 frequently needs cleaning.
When the fluidity of the anisotropic conductive film 3 is small, the anisotropic conductive film 3 tends to remain between the bump 11 and the wiring pattern 21 when the semiconductor chip 1 is heated and pressed by the thermo-compression tool 51. There occurs a variation in connection resistance of the bumps 11. Particularly when the substrate 2 has a warp, the anisotropic conductive film and the substrate fail to properly join each other, and some of the bumps 11 and the wiring pattern 21 suffer from a point contact as represented by a point contact area 39. In extreme cases, no electrical connection is established between the bump 11 and the wiring pattern 21.
The present invention resolves the above conventional problems. It is an object of the present invention to provide an anisotropic conductive film which reliably assures electrical connection between a substrate and a semiconductor chip and prevents the anisotropic conductive film from clinging to a thermo-compression tool, thereby permitting manufacturing steps to be easily managed.
It is another object of the present invention to provide a semiconductor device incorporating the anisotropic conductive film.
To achieve the above objects, as recited in accordance with one exemplary embodiment of the present invention, an anisotropic conductive film of the present invention, which bonds a semiconductor chip to a substrate while serving as an electrically conductive medium between the semiconductor chip and the substrate, includes in lamination a first layer including at least one layer structure having electrically conductive particles, and a second layer including at least one layer structure having a fluidity higher than the fluidity of the first layer.
When heated, the anisotropic conductive film of claim 1, as constructed, creates a different fluidity between the first layer and the second layer. With the first layer less fluid and thus higher in hardness, the anisotropic conductive film is prevented from flowing out from between the semiconductor chip and the substrate when the semiconductor chip is bonded to the substrate by thermo-compression. The number of electrically conductive particles interposed between the electrodes of the semiconductor chip and the electrodes of the substrate is thus increased. On the other hand, with the second layer being more fluid and softer than the first layer, the anisotropic conductive film easily flows outward from between the semiconductor chip and the substrate when the semiconductor chip is bonded by thermo-compression to the substrate. This arrangement forms the fillet clinging to the sides of the semiconductor chip without impeding the contact between the chip electrodes and the substrate electrodes.
Compared to the amount of resin flowing out in the conventional anisotropic conductive film, the amount of resin flowing out from between the semiconductor chip and the substrate is reduced. This arrangement prevents the anisotropic conductive film from clinging to the thermo-compression tool. As a result, the mechanical bond between the semiconductor chip and the substrate is securely maintained while the reliability of the electrical connection therebetween is increased. The management of the bonding step of the semiconductor chip to the substrate is simplified.
In order to reduce the amount of the anisotropic conductive film clinging to the sides of the semiconductor chip, the thickness of the above anisotropic conductive film is preferably equal to the thickness of the conventional film. If the density of the electrically conductive particles contained in the electrically conductive particle containing layer is equal to that of the conventional anisotropic conductive film, the number of the electrically conductive particles per unit volume will be smaller than that in the conventional anisotropic conductive film. The conductivity between the semiconductor chip and the substrate decreases. The density of the electrically conductive particles is preferably slightly higher than that in the conventional anisotropic conductive film.
In an anisotropic conductive film in accordance with a secondary exemplary embodiment of the present invention, the second layer is formed over first layer.
In the anisotropic conductive film in accordance with the above exemplary embodiment, the second layer is interposed between the first layer and both the active-element formation surface of the semiconductor chip and the wiring pattern formation surface of the substrate. This arrangement reduces the occurrence of direct contact of the electrically conductive particles contained in the first layer to the area of the semiconductor chip having no bumps formed in the active-element formation surface thereof and to the area of the substrate having no wiring pattern formed in the wiring pattern formation surface thereof. Even if metallic particles having sharp irregularities, such as nickel particles as the electrically conductive particles, are employed, the possibility of damaging the active-element formation surface and the wiring pattern formation surface is reduced. As a result, the candidate material range of the electrically conductive particles is expanded.
With two second layers respectively placed into contact with the substrate and the semiconductor chip, the more fluid second layers flows compliantly along the irregularities and warps of the active-element formation surface and the wiring pattern formation surface, thereby allowing the anisotropic conductive film, the semiconductor chip, and the substrate to tightly bond to each other. As a result, the mechanical bond between the semiconductor chip and the substrate is securely maintained while the reliability of the electrical connection therebetween is increased.
In an anisotropic conductive film in accordance with a third exemplary embodiment of the present invention, the first layer is interposed between the two second layers.
In the anisotropic conductive film constructed as recited in accordance with the above exemplary embodiment, the second layers are respectively interposed between the first layer and the active-element formation surface of the semiconductor chip and between the first layer and the wiring formation surface of the substrate. This arrangement reduces the occurrence of direct contact of the electrically conductive particles contained in the first layer to the area of the semiconductor chip having no bumps formed in the active-element formation surface and to the area of the substrate having no wiring pattern formed in the wiring pattern formation surface. Even if metallic particles having sharp irregularities, such as nickel particles as the electrically conductive particles, are employed, the possibility of damaging the active-element formation surface and the wiring pattern formation surface is reduced. As a result, the candidate material range of the electrically conductive particles is expanded.
In an anisotropic conductive film in accordance with a fourth exemplary embodiment of the present invention, the first layer is thicker than the second layer.
The anisotropic conductive film in accordance with the above exemplary embodiment keeps the electrically conductive particles contained in the first layer separated from each other by proper spacing, thereby preventing the bumps of the semiconductor chip from shorting each other with electrically conductive particles connected in series between the bumps. Furthermore, the amount of anisotropic conductive film clinging to the sides of the semiconductor chip is reduced to a minimum required amount.
In an anisotropic conductive film in accordance with a fifth exemplary embodiment of the present invention, the first layer is fabricated of a material having a low fluidity.
The anisotropic conductive film in accordance with the above exemplary embodiment reliably controls an excess flow of electrically conductive particles.
In an anisotropic conductive film in accordance with a sixth exemplary embodiment of the present invention, the highly fluid material has a lower density of the electrically conductive particles than the less fluid material.
In the anisotropic conductive film in accordance with the above exemplary embodiment, the highly fluid material also contributes to electrical connection between the semiconductor chip and the substrate, thereby assuring electrical connection.
In an anisotropic conductive film in accordance with a seventh exemplary embodiment of the present invention, a band-like body having a fluidity lower than the fluidity of the second layer is arranged on the periphery of at least one of the first layer and the second layer.
In the anisotropic conductive film in accordance with the above exemplary embodiment, the band-like body restricts an excess flow of the second layer, thereby preventing an excessively large fillet from being produced.
A circuit board in accordance with an eighth exemplary embodiment of the present invention includes a semiconductor chip and a substrate with an anisotropic conductive film according to one in accordance with one of the above exemplary embodiments interposed therebetween.
The circuit board in accordance with the above exemplary embodiment prevents the anisotropic conductive film from excessively flowing out into the periphery around the semiconductor chip and clinging to the remaining area of the circuit board when the semiconductor chip is thermo-compression bonded. The semiconductor chip is reliably connected, forming a reliable circuit board.
Electronic equipment in accordance with the ninth exemplary embodiment of the present invention includes a circuit board according in accordance with the eight exemplary embodiment.
Since the electronic equipment in accordance with the above exemplary embodiment includes a highly reliable circuit board with the semiconductor chip mounted, the electronic equipment itself becomes reliable.
A semiconductor device in accordance with the tenth exemplary embodiment of the present invention includes a substrate on which a semiconductor chip is mounted through an anisotropic conductive film, wherein the anisotropic conductive film is fabricated by laminating a first layer including at least one layer structure having electrically conductive particles, and a second layer including at least one layer structure having a fluidity higher than the fluidity of the first layer.
When heated, the semiconductor device in accordance with the above exemplary embodiment creates a different fluidity between the first layer and the second layer. With the first layer less fluid and thus higher in hardness, the anisotropic conductive film is prevented from flowing out from between the semiconductor chip and the substrate when the semiconductor chip is bonded to the substrate by thermo-compression. The number of electrically conductive particles between the electrodes of the semiconductor chip and the electrodes of the substrate is thus increased. On the other hand, with the second layer being more fluid and softer than the first layer, the anisotropic conductive film easily flows outward from between the semiconductor chip and the substrate when the semiconductor chip is bonded by thermo-compression to the substrate. This arrangement forms the fillet clinging to the sides of the semiconductor chip without impeding the contact between the chip electrodes and the substrate electrodes.
In a semiconductor device in accordance with the eleventh exemplary embodiment of the present invention, the second layer is formed over first layer.
In the semiconductor device in accordance with the above exemplary embodiment, the second layer achieves two purposes of assuring the mechanical bond and the electrical connection of the semiconductor chip to the substrate, when the semiconductor chip is thermo-compression bonded to the substrate. Specifically, the second layer fills the space surrounding the active-element formation surface of the semiconductor chip while forming the fillet on the sides of the semiconductor chip for enhancing the bond of the semiconductor chip, and interposes the electrically conductive particles between the semiconductor chip and the substrate for assuring the electrical connection therebetween.
As for materials, the above-referenced substrate may be an organic-material based substrate such as a plastic substrate or a flexible substrate, or an inorganic-material based substrate such as a ceramic substrate.